/**
 *Given a linked list, swap every two adjacent nodes and return its head.
 *For example,Given 1->2->3->4, you should return the list as 2->1->4->3.
 *Your algorithm should use only constant space. You may not modify the values in the list,
 * only nodes itself can be changed. 
 */
/**
 * @author Ivan
 *
 */
package problem24._Swap_Nodes_in_Pairs;